This invention relates to a process for producing self-supporting compact bodies for use in a metallurgical process. Such bodies are made of particulate (fine grained) materials containing metal oxides, fine-grained caking coal and optionally other non-caking materials. The non-caking materials, (e.g., metal oxides and other non-caking materials) are highly heated and mixed with the caking coal and the mixture is briquetted when the coal is in a plastic state.
Self-supporting compact bodies for use in a metallurgical process are briquettes made from fine-grained or dust-like starting materials which can be reduced to metals and smelted without a separate or added supply of solid fuel. In this connection it is a special advantage that fine-grained ore, flotation concentrate, converter dust or the like, together with the fuel required for the reduction process, can be processed to form uniform particles, which are desirable for the reduction or smelting process and result in a charge having a substantially uniform particle size. During transportation, storage and further processing, the compact bodies must meet high requirements as regards crushing strength and abrasion resistance and they must have good dimensional stability at elevated temperatures.
Two basically different processes are known for briquetting fine-grained materials contining metal oxides. In the so-called cold-briquetting process, the fine-grained materials are mixed with binders, such as tar, bitumen or the like, in a solid or liquid state, without being preheated to elevated temperatures, and are then compacted and after-treated to coke the binders. In the second process, caking coal is used as a binder rather than tar, bitumen, etc. Certain kinds of coal soften and form a solid decomposition product generally known as coke at temperatures in the range from 350.degree.-550.degree.C. When softened, such coal can bond non-caking materials so that so-called hot briquettes can be made at suitable temperatures.
The quality of the hot briquettes depends not only on the nature of the starting materials and the conditions (such as temperature and pressure during compacting), but also on a sufficiently fast and uniform mixing of the components. Since fine-grained caking coal cannot be heated directly to the required temperature (because of its tendency to soften, stick and cake), it is usual to heat the non-caking components to a temperature, which is sufficiently above the required temperature so that a mixture at the required temperature will be obtained when the heated non-caking components are mixed with the caking coal which itself may be preheated. A suitable heat balance taking into consideration the quantities and specific heats of the components and the temperature to which the caking coal can be preheated allows calculation of the extent to which the non-caking components must be overheated.
There can be limits to the overheating of the non-caking materials and, in this case, the caking coal must be preheated close to its softening point. Mixing must be performed rapidly, intensely, and uniformly so as to avoid excessive heating of the caking coal in contact with the overheated non-caking components. Excessive heating of the caking coal causes it to lose its caking property and the ability to obtain a highly homogeneous mixture for a production of high-grade hot briquettes. Additionally, the mixing operation must not last too long because this adversely affects the caking property of the coal. For these reasons, a satisfactory mixing is required in view of the quality of the compact bodies and for the optimum utilization of the caking capacity of the caking coal.
In a known process for mixing high-temperature, fine-grained materials with caking coal, the mixing operation is performed in a carrier gas stream from which a mixture of the components is separated and subsequently briquetted (printed German application No. 1,180,344). In a development of this process, the starting components are heated to different temperatures because the non-softening component and the caking coal are introduced into a hot gas stream in succession, in its direction of flow. The non-softening component is separated from the hot gas stream before the caking coal is introduced and separately separated. Only thereafter are the separately separated components mixed in a separate unit (German Pat. No. 1,696,509). Finally, a mixer has been disclosed which is used to mix an overheated, non-caking component with preheated caking coal, whereafter the mixture can be cooled to a predetermined extent from an adjusted mixed temperature. The mixer has screws which rotate in the same sense and have a lens-shaped profile for a good revolving and continuous feeding of the material to be briquetted (German Pat. No. 1,252,623).
It has now been found that the known processes and apparatus do not produce satisfactory mixtures if the specific gravity of the non-caking component is much higher than that of the caking coal or if the non-caking component exceeds certain proportions. At the beginning of mixing the caking coal, depending on its preheating, is at a temperature, which is more or less below its softening point. It thus lacks at this point in time an adequate ability to bond the hot particles, which have a higher specific gravity. In addition, the coal swells as it is heated and releases volatile constitutents so that it temporarily has a lower density than in the cold and the difference between the specific gravities is even increased temporarily.
Because of this difference in specific gravities, accelerating forces in the mixer (gravitational field, cyclone) during the mixing operation result in a certain segregation into particles having lower and higher specific gravities, respectively, and this segregation is not eleiminated as the mixing operation proceeds. Zones or layers are thus formed in the mixture and have an adverse effect not only on the uniformity of the mixture but also on the temperature equalization.